linux-kernel - Re: [PATCH 00/25] mm: Page fault accounting cleanups

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Message-ID: <87imfqecjx.fsf@mpe.ellerman.id.au>
Date:   Wed, 17 Jun 2020 10:55:14 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        openrisc@...ts.librecores.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 00/25] mm: Page fault accounting cleanups

Linus Torvalds <torvalds@...ux-foundation.org> writes:
> On Mon, Jun 15, 2020 at 3:16 PM Peter Xu <peterx@...hat.com> wrote:
>> This series tries to address all of them by introducing mm_fault_accounting()
>> first, so that we move all the page fault accounting into the common code base,
>> then call it properly from arch pf handlers just like handle_mm_fault().
>
> Hmm.
>
> So having looked at this a bit more, I'd actually like to go even
> further, and just get rid of the per-architecture code _entirely_.

<snip>

> One detail worth noting: I do wonder if we should put the
>
>     perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
>
> just in the arch code at the top of the fault handling, and consider
> it entirely unrelated to the major/minor fault handling. The
> major/minor faults fundamnetally are about successes. But the plain
> PERF_COUNT_SW_PAGE_FAULTS could be about things that fail, including
> things that never even get to this point at all.

Yeah I think we should keep it in the arch code at roughly the top.

If it's moved to the end you could have a process spinning taking bad
page faults (and fixing them up), and see no sign of it from the perf
page fault counters.

cheers